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Description of the Related Art
The present invention relates to access networks. One such access network, a passive optical network (PON) 100, is illustrated in FIG. 1. Data flows from the external feeder network 110 through the distribution network 100 to the subscribers via the optical network units (ONUs) 1 through N. Data flow in this direction is referred to as the downstream direction. The feeder trunks are terminated at the host digital terminal (HDT) 115. The HDT 115 cross connects the feeder payload to the appropriate PON transmitter which in turn transmits it to all the ONUs subtended off that given PON. The ONUs provide the line card functionality to support the subscriber(s). The ONUs may either support multiple buildings or be dedicated to a single building.
The data flow from the subscriber via the ONUs to the feeder network. This is called the upstream direction. The subscribers send data to the ONUs which in turn transmit the data to the HDT over the PON. The HDT cross connects the PON payload to the appropriate feeder trunk.
A PON network traditionally has the following characteristics:
Both the upstream and downstream are digital.
The downstream is asynchronous, in both phase and frequency.
The downstream data rate is constant.
The downstream is sourced from a single distribution node.
The downstream optical power is equally split N ways to serve N network nodes.
The upstream is asynchronous in phase, and synchronous in frequency.
The upstream data rate from all nodes is the same.
The upstream optical power is equally combined N ways from the N network nodes to the distribution node.
With reference to FIG. 5, of particular interest is the method in which timing is distributed in the traditional PON. The traditional PON sources all its timing from the downstream direction. The Headend PON Termination Modules (HPTM) transmit an optical data stream at a fixed rate. The downstream data is terminated at the receiver 505 in the ONU 510. A PLL 515 in the ONU recovers the clock and retimes the data. The recovered clock acts as the master clock for all operations in the ONU, including the data rate for the line cards and the data rate for the upstream transmission as well. Because the data from the line cards must interface ultimately with the feeder line rate, the clock rate generally must be synchronized with the feeder data rate.
Although the PON architecture has a number of advantages over other architectures (such as point to point, mesh, active star, bus topologies) it does not have a good upgrade path. One prior art approach to upgrading a PON is a wave division multiplex (WDM) overlay as shown in FIG. 2. The salient features of a WDM overlay are:
1. The traffic carried by the second wavelength shares the same fiber and splitters.
2. Each ONU requires a WDM filter 205 to filter out the unwanted wavelength(s).
The disadvantages of this upgrade are:
1. The ONUs that are not upgraded must nevertheless have a filter installed. This is expensive and also affects service.
2. The ONUs that are upgraded have essentially the same end to end loss as those that are not upgraded. This makes it extremely difficult to design the transmitters and receivers for the higher speed upgraded PON. As data rates increase, receiver sensitivity decreases.
The designer of the link can either increase the power of the high speed transmitter or maintain the receiver sensitivity by using more sophisticated receiver circuitry. Usually both are done. If the transmitter power is increased, then crosstalk may be an issue requiring better, more expensive filters. Worse yet, if the PON is a single fiber PON, then the reflection budget is more difficult to manage, requiring yet more expensive filters. If the receiver sensitivity is maintained or increased, more sophisticated techniques and components are required. The net result, in any case, is an increase in cost and perhaps a longer design cycle. Furthermore, as speeds increase, the limits of technology to compensate only by changing the receiver or transmitter is reached.
A good upgrade path should allow for the data rate of the PON to be increased within the following constraints:
the upgrade can be done on a per network node basis. That is, one network node should be able to communicate at higher data rates without having to upgrade all other nodes on the same PON;
the upgrade should not affect service for those network nodes that are not upgraded;
no new cable is required. Slight modifications at the splitter may be permissible depending on the cost of those changes;
minimal changes to cards at the HDT and at the upgraded network node, optimally only the electro-optic printed circuit boards (PCB);
no changes at the non-upgraded network nodes.
Definition of Terms
DPACxe2x80x94Digital Phase Acquisition Circuit
HDTxe2x80x94Host Digital Terminal
HPTMxe2x80x94Headend PON Termination Modules
ONUxe2x80x94Optical Network Unit.
PONxe2x80x94Passive Optical Network
This invention embodies several methods and the associated hardware and software necessary for a Passive Optical Network (PON) to operate at significantly higher data rates than the prior art Passive Optical Networks.